Whenever the President finds that the amount of petroleum and petroleum products moving in interstate commerce is so limited as to be the cause, in whole or in part, of a lack of parity between supply (including imports and reasonable withdrawals from storage) and consumptive demand (including exports and reasonable additions to storage) resulting in an undue burden on or restriction of interstate commerce in petroleum and petroleum products, he shall by proclamation declare such finding, and thereupon the provisions of section 715b of this title shall be inoperative until such time as the President shall find and by proclamation declare that the conditions which gave rise to the suspension of the operation of the provisions of such section no longer exist. If any provision of this section or the application thereof shall be held to be invalid, the validity of application of section 715b of this title shall not be affected thereby.